/****************************************************************************
 *
 * ftlcdfil.h
 *
 *   FreeType API for color filtering of subpixel bitmap glyphs
 *   (specification).
 *
 * Copyright (C) 2006-2019 by
 * David Turner, Robert Wilhelm, and Werner Lemberg.
 *
 * This file is part of the FreeType project, and may only be used,
 * modified, and distributed under the terms of the FreeType project
 * license, LICENSE.TXT.  By continuing to use, modify, or distribute
 * this file you indicate that you have read the license and
 * understand and accept it fully.
 *
 */

#ifndef FTLCDFIL_H_
#define FTLCDFIL_H_

#include <ft2build.h>
#include FT_FREETYPE_H
#include FT_PARAMETER_TAGS_H

#ifdef FREETYPE_H
#error "freetype.h of FreeType 1 has been loaded!"
#error "Please fix the directory search order for header files"
#error "so that freetype.h of FreeType 2 is found first."
#endif

FT_BEGIN_HEADER

/**************************************************************************
 *
 * @section:
 *   lcd_rendering
 *
 * @title:
 *   Subpixel Rendering
 *
 * @abstract:
 *   API to control subpixel rendering.
 *
 * @description:
 *   FreeType provides two alternative subpixel rendering technologies.
 *   Should you define `FT_CONFIG_OPTION_SUBPIXEL_RENDERING` in your
 *   `ftoption.h` file, this enables patented ClearType-style rendering.
 *   Otherwise, Harmony LCD rendering is enabled.  These technologies are
 *   controlled differently and API described below, although always
 *   available, performs its function when appropriate method is enabled
 *   and does nothing otherwise.
 *
 *   ClearType-style LCD rendering exploits the color-striped structure of
 *   LCD pixels, increasing the available resolution in the direction of
 *   the stripe (usually horizontal RGB) by a factor of~3.  Using the
 *   subpixels coverages unfiltered can create severe color fringes
 *   especially when rendering thin features.  Indeed, to produce
 *   black-on-white text, the nearby color subpixels must be dimmed
 *   equally.
 *
 *   A good 5-tap FIR filter should be applied to subpixel coverages
 *   regardless of pixel boundaries and should have these properties:
 *
 *   1. It should be symmetrical, like {~a, b, c, b, a~}, to avoid
 *      any shifts in appearance.
 *
 *   2. It should be color-balanced, meaning a~+ b~=~c, to reduce color
 *      fringes by distributing the computed coverage for one subpixel to
 *      all subpixels equally.
 *
 *   3. It should be normalized, meaning 2a~+ 2b~+ c~=~1.0 to maintain
 *      overall brightness.
 *
 *   Boxy 3-tap filter {0, 1/3, 1/3, 1/3, 0} is sharper but is less
 *   forgiving of non-ideal gamma curves of a screen (and viewing angles),
 *   beveled filters are fuzzier but more tolerant.
 *
 *   Use the @FT_Library_SetLcdFilter or @FT_Library_SetLcdFilterWeights
 *   API to specify a low-pass filter, which is then applied to
 *   subpixel-rendered bitmaps generated through @FT_Render_Glyph.
 *
 *   Harmony LCD rendering is suitable to panels with any regular subpixel
 *   structure, not just monitors with 3 color striped subpixels, as long
 *   as the color subpixels have fixed positions relative to the pixel
 *   center.  In this case, each color channel is then rendered separately
 *   after shifting the outline opposite to the subpixel shift so that the
 *   coverage maps are aligned.  This method is immune to color fringes
 *   because the shifts do not change integral coverage.
 *
 *   The subpixel geometry must be specified by xy-coordinates for each
 *   subpixel. By convention they may come in the RGB order: {{-1/3, 0},
 *   {0, 0}, {1/3, 0}} for standard RGB striped panel or {{-1/6, 1/4},
 *   {-1/6, -1/4}, {1/3, 0}} for a certain PenTile panel.
 *
 *   Use the @FT_Library_SetLcdGeometry API to specify subpixel positions.
 *   If one follows the RGB order convention, the same order applies to the
 *   resulting @FT_PIXEL_MODE_LCD and @FT_PIXEL_MODE_LCD_V bitmaps.  Note,
 *   however, that the coordinate frame for the latter must be rotated
 *   clockwise.  Harmony with default LCD geometry is equivalent to
 *   ClearType with light filter.
 *
 *   As a result of ClearType filtering or Harmony rendering, the
 *   dimensions of LCD bitmaps can be either wider or taller than the
 *   dimensions of the corresponding outline with regard to the pixel grid.
 *   For example, for @FT_RENDER_MODE_LCD, the filter adds 2~subpixels to
 *   the left, and 2~subpixels to the right.  The bitmap offset values are
 *   adjusted accordingly, so clients shouldn't need to modify their layout
 *   and glyph positioning code when enabling the filter.
 *
 *   The ClearType and Harmony rendering is applicable to glyph bitmaps
 *   rendered through @FT_Render_Glyph, @FT_Load_Glyph, @FT_Load_Char, and
 *   @FT_Glyph_To_Bitmap, when @FT_RENDER_MODE_LCD or @FT_RENDER_MODE_LCD_V
 *   is specified.  This API does not control @FT_Outline_Render and
 *   @FT_Outline_Get_Bitmap.
 *
 *   The described algorithms can completely remove color artefacts when
 *   combined with gamma-corrected alpha blending in linear space.  Each of
 *   the 3~alpha values (subpixels) must by independently used to blend one
 *   color channel.  That is, red alpha blends the red channel of the text
 *   color with the red channel of the background pixel.
 */

/**************************************************************************
 *
 * @enum:
 *   FT_LcdFilter
 *
 * @description:
 *   A list of values to identify various types of LCD filters.
 *
 * @values:
 *   FT_LCD_FILTER_NONE ::
 *     Do not perform filtering.  When used with subpixel rendering, this
 *     results in sometimes severe color fringes.
 *
 *   FT_LCD_FILTER_DEFAULT ::
 *     This is a beveled, normalized, and color-balanced five-tap filter
 *     with weights of [0x08 0x4D 0x56 0x4D 0x08] in 1/256th units.
 *
 *   FT_LCD_FILTER_LIGHT ::
 *     this is a boxy, normalized, and color-balanced three-tap filter with
 *     weights of [0x00 0x55 0x56 0x55 0x00] in 1/256th units.
 *
 *   FT_LCD_FILTER_LEGACY ::
 *   FT_LCD_FILTER_LEGACY1 ::
 *     This filter corresponds to the original libXft color filter.  It
 *     provides high contrast output but can exhibit really bad color
 *     fringes if glyphs are not extremely well hinted to the pixel grid.
 *     This filter is only provided for comparison purposes, and might be
 *     disabled or stay unsupported in the future. The second value is
 *     provided for compatibility with FontConfig, which historically used
 *     different enumeration, sometimes incorrectly forwarded to FreeType.
 *
 * @since:
 *   2.3.0 (`FT_LCD_FILTER_LEGACY1` since 2.6.2)
 */
typedef enum FT_LcdFilter_ {
	FT_LCD_FILTER_NONE = 0,
	FT_LCD_FILTER_DEFAULT = 1,
	FT_LCD_FILTER_LIGHT = 2,
	FT_LCD_FILTER_LEGACY1 = 3,
	FT_LCD_FILTER_LEGACY = 16,

	FT_LCD_FILTER_MAX /* do not remove */

} FT_LcdFilter;

/**************************************************************************
 *
 * @function:
 *   FT_Library_SetLcdFilter
 *
 * @description:
 *   This function is used to apply color filtering to LCD decimated
 *   bitmaps, like the ones used when calling @FT_Render_Glyph with
 *   @FT_RENDER_MODE_LCD or @FT_RENDER_MODE_LCD_V.
 *
 * @input:
 *   library ::
 *     A handle to the target library instance.
 *
 *   filter ::
 *     The filter type.
 *
 *     You can use @FT_LCD_FILTER_NONE here to disable this feature, or
 *     @FT_LCD_FILTER_DEFAULT to use a default filter that should work well
 *     on most LCD screens.
 *
 * @return:
 *   FreeType error code.  0~means success.
 *
 * @note:
 *   This feature is always disabled by default.  Clients must make an
 *   explicit call to this function with a `filter` value other than
 *   @FT_LCD_FILTER_NONE in order to enable it.
 *
 *   Due to **PATENTS** covering subpixel rendering, this function doesn't
 *   do anything except returning `FT_Err_Unimplemented_Feature` if the
 *   configuration macro `FT_CONFIG_OPTION_SUBPIXEL_RENDERING` is not
 *   defined in your build of the library, which should correspond to all
 *   default builds of FreeType.
 *
 * @since:
 *   2.3.0
 */
FT_EXPORT(FT_Error)
FT_Library_SetLcdFilter(FT_Library library, FT_LcdFilter filter);

/**************************************************************************
 *
 * @function:
 *   FT_Library_SetLcdFilterWeights
 *
 * @description:
 *   This function can be used to enable LCD filter with custom weights,
 *   instead of using presets in @FT_Library_SetLcdFilter.
 *
 * @input:
 *   library ::
 *     A handle to the target library instance.
 *
 *   weights ::
 *     A pointer to an array; the function copies the first five bytes and
 *     uses them to specify the filter weights in 1/256th units.
 *
 * @return:
 *   FreeType error code.  0~means success.
 *
 * @note:
 *   Due to **PATENTS** covering subpixel rendering, this function doesn't
 *   do anything except returning `FT_Err_Unimplemented_Feature` if the
 *   configuration macro `FT_CONFIG_OPTION_SUBPIXEL_RENDERING` is not
 *   defined in your build of the library, which should correspond to all
 *   default builds of FreeType.
 *
 *   LCD filter weights can also be set per face using @FT_Face_Properties
 *   with @FT_PARAM_TAG_LCD_FILTER_WEIGHTS.
 *
 * @since:
 *   2.4.0
 */
FT_EXPORT(FT_Error)
FT_Library_SetLcdFilterWeights(FT_Library library, unsigned char *weights);

/**************************************************************************
 *
 * @type:
 *   FT_LcdFiveTapFilter
 *
 * @description:
 *   A typedef for passing the five LCD filter weights to
 *   @FT_Face_Properties within an @FT_Parameter structure.
 *
 * @since:
 *   2.8
 *
 */
#define FT_LCD_FILTER_FIVE_TAPS 5

typedef FT_Byte FT_LcdFiveTapFilter[FT_LCD_FILTER_FIVE_TAPS];

/**************************************************************************
 *
 * @function:
 *   FT_Library_SetLcdGeometry
 *
 * @description:
 *   This function can be used to modify default positions of color
 *   subpixels, which controls Harmony LCD rendering.
 *
 * @input:
 *   library ::
 *     A handle to the target library instance.
 *
 *   sub ::
 *     A pointer to an array of 3 vectors in 26.6 fractional pixel format;
 *     the function modifies the default values, see the note below.
 *
 * @return:
 *   FreeType error code.  0~means success.
 *
 * @note:
 *   Subpixel geometry examples:
 *
 *   - {{-21, 0}, {0, 0}, {21, 0}} is the default, corresponding to 3 color
 *   stripes shifted by a third of a pixel. This could be an RGB panel.
 *
 *   - {{21, 0}, {0, 0}, {-21, 0}} looks the same as the default but can
 *   specify a BGR panel instead, while keeping the bitmap in the same
 *   RGB888 format.
 *
 *   - {{0, 21}, {0, 0}, {0, -21}} is the vertical RGB, but the bitmap
 *   stays RGB888 as a result.
 *
 *   - {{-11, 16}, {-11, -16}, {22, 0}} is a certain PenTile arrangement.
 *
 *   This function does nothing and returns `FT_Err_Unimplemented_Feature`
 *   in the context of ClearType-style subpixel rendering when
 *   `FT_CONFIG_OPTION_SUBPIXEL_RENDERING` is defined in your build of the
 *   library.
 *
 * @since:
 *   2.10.0
 */
FT_EXPORT(FT_Error)
FT_Library_SetLcdGeometry(FT_Library library, FT_Vector sub[3]);

/* */

FT_END_HEADER

#endif /* FTLCDFIL_H_ */

/* END */
